A micro-porous structure having an open flow field shape rather than an existing channel-shaped path may be inserted into a separator of a fuel cell stack to increase the reaction efficiency of the fuel cell stack. One advantage of the existing traditional channel separator is that reacting-gas and coolant passages are formed by stacking anode/cathode separators defining channels that are the flow passages of the reacting gas, so that it is possible to simplify the structure of a fuel cell. However, surface pressure in the existing traditional channel separator may become non-uniform due to the channel/land shape of the path, so that electric resistance increases. Furthermore, the structure of the gas diffusion layer may be destroyed due to the concentration of excessive stress on a land portion, so that the diffusion ability of the reacting gas is deteriorated.
In a porous-body path separator, however, if a micro-porous structure such as metal/carbon foam or a wire mesh is inserted into the reactive surface instead of the existing channel-shaped path, the flow of the reaction gas and produced water is facilitated and the GDL (Gas Diffusion Layer) is uniformly compressed to distribute surface pressure. Consequently, electric resistance may be minimized and improvement in performance of the fuel cell may be maximized. The existing micro-porous path structure is problematic, however, in that manufacturing costs thereof are high, and a weight and a volume thereof are increased, thus leading to a reduction of mass-productivity.
FIG. 1 is a sectional view illustrating a conventional separator for a porous body structure, and FIG. 2 is a top view illustrating the conventional separator when viewed from a vertical direction. The fuel cell includes a membrane-electrode assembly (MEA) 10 at an intermediate position and a gas diffusion layer (GM) 30, with a porous panel 50 and a separator 70 being coupled thereto. The porous panel 50 of the conventional separator includes a plurality of through holes 54 that are repeatedly formed at regular intervals on left and right sides of each of linear uneven lines 52 perpendicular to a longitudinal direction (gas flow direction). By repeating these linear uneven lines 52 in the gas flow direction (longitudinal direction), the diffusion of gas is increased in the reactive surface. Particularly in a high-current section having high fuel consumption, a flow rate is correspondingly increased, so that the effect of flow resistance is increased by the shape of a porous body and thereby the effect of the porous body is maximized.
Since reacting gas passing through the through holes formed on one side is blocked by a wall surface of a channel, the reacting gas flows in the same manner as G2 in a widthwise direction where adjacent through holes are arranged, so as to pass to the next channel. Therefore, the repetition of such a flow causes a zigzag flow, so that it is possible to increase the ability to diffuse the reacting gas.
When analyzing a driving pattern of a vehicle equipped with the fuel cell stack, 70% or more of an operating region consists of low/middle current where the flow rate of the reaction gas is small. In order to maximize the effect of formed porous body, the porous body should fulfill its effect in a low/middle current section as well as a high current section where the diffusion of the fuel is important.
Since the basic concept of the porous body consists in flow disturbance through the through holes that are formed at regular intervals on a side of the channel to cross the channel, design factors having great effect on the porous body are a width a of each through hole in the porous body and an interval b between the through holes.
If the width a of the hole is greater than the interval b between the adjacent holes, an overlapping section is created between adjacent channels. In this case, flow resistance is not large in the low/middle current section predominantly occupying the operating region of the fuel cell, so that no zigzag flow occurs and most of the fluid escapes through the overlapping sections of the repeated holes, as in G1. Therefore, this is excellent in terms of the concept of the formed porous body, but reduces a real gain of the porous body in the fuel cell vehicle in consideration of a driving pattern of a driver who drives the real fuel cell vehicle.
In contrast, if the interval b between the adjacent holes is greater than the width a of the hole, the overlapping section is eliminated between adjacent channels. Therefore, the flow resistance increases regardless of the flow intensity, so that the zigzag flow is continuously generated as in G2 and the effect of improving the diffusion ability of the porous body is maximized. As the interval between the holes increases, however, the length of the path increases throughout a whole operating region and a differential pressure is entirely increased. Therefore, this increases the number of auxiliary components for driving the fuel cell and thereby causes a reduction in the efficiency of the fuel cell system.
Furthermore, as the interval between the holes increases, an excessive amount of liquid droplets may stay between the holes where flow is relatively weak, thus causing a reduction in the stability of a low-temperature operation and a deterioration in cold-startability of a vehicle.
Therefore, an uneven structure for a separator has been developed, which a capable of maximizing a gas diffusion effect of a porous body regardless of a flow intensity while reducing a differential pressure in the separator and improving a water discharge ability of the separator.
The foregoing is designed merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.